The present invention relates to clock circuits and, more particularly, to an exponential clock circuit useful in association with an electronic musical instrument for producing a tone signal having a frequency sweeping one or more musical intervals at an exponential rate.
Copending application Ser. No. 961,222, filed Nov. 16, 1978, in the name of Glenn Gross, which is incorporated herein by reference, discloses a tone generator for an electronic musical instrument adapted for producing various musical effects such as portamento and glissando, both of which comprehend continuously sweeping the frequency of a tone signal through one or more selected musical intervals. The disclosed tone generator includes an up-down counter operated for developing a continuously stepped output signal for controlling the divisor of a programmable frequency divider at whose output is developed the swept tone signals. The up-down counter is clocked by the output of a rate multiplier which has its program inputs connected to the output of the up-down counter and its clock input for receiving a constant repetition rate clock signal from a rate control clock. The effect of the rate multiplier is to compensate system operation such that corresponding musical intervals, e.g. semitones, are swept in equal time intervals and at a linear rate regardless of the relative position of the interval in the musical scale. This compensation effect, as a result of which, a given number of musical intervals will always be swept in a corresponding number of equal time intervals, is achieved as a consequence of coupling to the rate multiplier an equal number of clock pulses for each musical interval swept, the rate multiplier coupling the clock pulses to the clock input of the counter at a reduced repetition rate determined according to the relative position of a swept interval in the musical scale.
In order to minimize the abruptness of the transition occurring at the end of a sweep, it has been found desirable to sweep the frequency of the tone signal at an exponential rather than a linear rate. Such may be accomplished in association with the tone generatior described in the previously mentioned copending application by coupling clock pulses to the rate multiplier which have a repetition rate varying exponentially with time rather than at a constant rate as in the linear case. However, in order to maintain the previously described time correspondence wherein a given number of musical intervals are swept in a corresponding number of equal time intervals, appropriate steps need to be taken to insure that the number of clock pulses coupled to the rate multiplier is directly proportional to the number of musical intervals to be swept. Also, it is desirable that the rate of change of the exponentially varying tone signals should remain substantially constant at the beginning of a sweep regardless of the number of musical intervals to be swept and of the position of the swept intervals in the musical scale. Similar considerations also apply to the rate of change of the tone signals at the end of a sweep.
Yet another consideration involves the condition wherein the musical instrument is operated for modifying a previously initiated frequency sweep prior to its completion. For example, assume that the musical instrument has been operated for and is in the process of generating a tone signal having a frequency sweeping between two values f.sub.1 and f.sub.2. Now, if prior to the completion of the frequency sweep, the instrument is operated for producing a tone signal having a frequency f.sub.3, it would be desirable to provide a capability for enabling the tone signal for being suitably swept from the frequency at which the initial sweep was interrupted to the newly designated frequency f.sub.3. The latter sweep to the newly designated frequency should be accomplished in a manner maintaining the previously discussed time correspondence and should also be characterized by an exponential rate of change at its initiation and completion corresponding to the previously mentioned constant values.